#include "cstring"
#include "cstdio"
#include "string"
#include "iostream"
using namespace std;
#define maxn 23000
int n,r[maxn],tmp[maxn];
template <class T>
inline bool read(T &ret)
{char c;int sgn;if(c=getchar(),c==EOF) return 0; //EOFwhile(c!='-'&&(c<'0'||c>'9')) c=getchar();sgn=(c=='-')?-1:1;ret=(c=='-')?0:(c-'0');while(c=getchar(),c>='0'&&c<='9') ret=ret*10+(c-'0');ret*=sgn;return 1;
}
struct Suffix
{int sa[maxn],rk[maxn],height[maxn];int t[maxn],t2[maxn],c[maxn],m;void init() {m=200;}int cmp(int *r,int a,int b,int l) {return r[a]==r[b]&&r[a+l]==r[b+l];}void build(){int i,k,p,*x=t,*y=t2;r[n++]=0;for (i=0; i<m; i++) c[i]=0;for (i=0; i<n; i++) c[x[i]=r[i]]++;for (i=1; i<m; i++) c[i]+=c[i-1];for (i=n-1; i>=0; i--) sa[--c[x[i]]]=i;for (k=1,p=1; k<n; k*=2,m=p){for (p=0,i=n-k; i<n; i++) y[p++]=i;for (i=0; i<n; i++) if (sa[i]>=k) y[p++]=sa[i]-k;for (i=0; i<m; i++) c[i]=0;for (i=0; i<n; i++)  c[x[y[i]]]++;for (i=1; i<m; i++) c[i]+=c[i-1];for (i=n-1; i>=0; i--) sa[--c[x[y[i]]]]=y[i];swap(x,y);p=1;x[sa[0]]=0;for (i=1; i<n; i++) x[sa[i]]=cmp(y,sa[i-1],sa[i],k)?p-1:p++;}n--;}void LCP(){int i,j,k=0;for (i=1; i<=n; i++) rk[sa[i]]=i;for (i=0; i<n; i++){if (k) k--;j=sa[rk[i]-1];while (r[i+k]==r[j+k]) k++;height[rk[i]]=k;}}bool judge(int len){int l=sa[1],r=sa[1];for(int i=2;i<=n;i++){if(height[i]<len){l=sa[i];r=sa[i];continue;}l=min(l,sa[i]);r=max(r,sa[i]);if(r-l>len) return true;}return false;}int BinarySearch(){int l=0,r=n,mid,ans=0;//注意ans=0，不然当n=1的时候返回的是没赋值的answhile(l<=r){int mid=l+(r-l)/2;if(judge(mid)) {ans=mid;l=mid+1;}else r=mid-1;}return ans;}
};
int main()
{//freopen("in.txt","r",stdin);//freopen("out1.txt","w",stdout);while(read(n)&&n){for(int i=0; i<n; i++) read(tmp[i]);for(int i=0; i<n-1; i++) {r[i]=tmp[i+1]-tmp[i]+90;}Suffix a;a.init();a.build();a.LCP();int ans=a.BinarySearch();if(ans<4) printf("0\n");else printf("%d\n",ans+1);}
}